Sugar's impact on cancer is often misunderstood

Sugar and cancer

Key points – the influence of sugar on cancer can be summarized as:

  • Higher body fat is linked to greater sugar consumption
  • Higher sugar consumption is linked to greater risks of developing type 2 diabetes, which is a recognized risk factor for many cancers and cancer progression
  • Even in non-diabetic people, a high sugar intake increases the body’s need for insulin. Higher insulin does appear to increase tumor growth in many cancers
  • Glucose, a form of sugar, directly feeds cell growth in some cancers
  • Sugar as a component of diets high in processed foods, may have an adverse effect on the balance of microbes in the intestines, which influences the immune system, which in turn has implications for treatment and management of cancer
  • Overall, though definitive answers cannot be given based on the current available evidence, those with cancer, or those looking to reduce their risk of developing cancer, would be wise to address the levels of sugar in their diet

People affected by cancer often ask if there are avoidable factors that may increase cancer risk or even affect the course of their disease. Amongst the hot topics in nutrition is the effect of sugar on health, and common questions raised by people with cancer are “does sugar feed cancer” or “how does sugar affect my cancer”? The answers are complex, and we will address the main themes in the science involved.

Sugar has a long complex history as a food component and commodity. Sugar was first described in ancient India over two thousand years ago, and consumption worldwide increased dramatically after it became exploited as an international commodity from the early 16th Century. As a food component in its simplest form, it does provide calories that the body uses for energy. It also provides a distinctive pleasurable dimension to many foods. It’s also surprising how often it does appear as an added ingredient in many common ‘savoury’ foods, such as vegetable products, sauces and processed meats.  In fact, when buying processed foods it’s better to assume that sugar has been added to it.

Sugar as a food substance

‘Sugar’ comes in many forms and it’s commonly thought of as the crystalized substance sucrose that it extracted from sugar cane or from sugar beet. Scientifically speaking, there are many different types of sugars, they are ‘saccharides’ , simple carbohydrates, and they also include lactose (milk sugar), fructose (fruit sugar), maltose (malt sugar) and glucose. Some are composed of a single saccharide unit, and they are described as monosaccharides, and some contain two base units and are termed disaccharides. All of them taste sweet to varying degrees, and all can provide energy to sustain life. The simplest, glucose, can be thought of as the common currency for living cells, of higher animals at least. In short, your cells generate energy by “burning” glucose, in the presence of oxygen, to create a molecule called ATP.  When you eat, your body breaks down (most) carbohydrates to make glucose, which can then be used for energy.  It is also possible for your body to “burn” fat (ketone bodies) when carbohydrates are not available, and that is the subject of another article, but the body will burn glucose first whenever it is available.

Sugary lollies

Disaccharide sugars such as lactose, maltose and sucrose are digested to release glucose and other saccharides for energy production. Sugars are also released from breaking down complex carbohydrates (starch) in foods such as rice, bread and potatoes. In this article, we’ll use the term sugar to describe sucrose and we’ll look at how glucose is handled in the body.

Sucrose and other simple sugars are rapidly digested in the human digestive system and then rapidly absorbed into the bloodstream to sustain a level of blood glucose, also often referred to as blood sugar. We’ll look at how this relates to cancer, and examine four main perspectives to the question: ”does sugar feed cancer?” Firstly, eating more sugar is linked to weight gain due to higher body fat and obesity, which is linked to higher cancer rates. Secondly higher sugar consumption is linked to developing type 2 diabetes which is also linked to increased cancer risk. These first two mechanisms are widely accepted and acknowledged in cancer research. Two further potential effects of sugar on cancer are still questioned: does sugar directly fuel the growth of cancer cells? Finally, how does dietary sugar affects the balance of microbes in the digestive system? The suggestion that sugar does directly feed cancer is still very hotly debated, and we’ll look at the debate and how individual people affected by cancer might consider this debate and navigate their own dietary path whilst living with cancer.

Sugar and Body Fat

Research looking at diet in different populations does link sugar consumption with weight gain, across age ranges and different ethnic and cultural groups. Obesity is more common in populations who consume more sugar, and looking at evidence over time for individual countries suggests weight gain is a steady feature as sugar consumption increases1. However, it can also be said that it’s the overall diet that contributes to obesity, as well as other individual food components, and lack of physical activity does play a part in rising obesity rates. There are also debates about the effects of some types of modified sugar ingredients, for example the use of high fructose corn syrup (HFCS) in processed foods and drinks. Feeding children a diet high in sugar does appear to increase obesity in later years, and this obesity is linked to higher adult cancer risk2. So, the ‘bottom line’ of research on sugar and body fat is that eating more sugar, processed foods and sweetened beverages is linked to obesity.

Unfortunately, obesity and higher body fat (adiposity) are risks for many cancers, including the most common cancers: endometrial, breast, colorectal, kidney, stomach, ovarian, prostate cancers and some blood cancers including myeloma. One elegant study published in 20143 looked at how many more cancers could be expected in one population as the average body mass index (BMI) was increased. Some cancers are so strongly influenced by body fat that the rates of them were predicted to increase dramatically. 40% of cases of cancer of the lining of the womb and 20% gall bladder cancers are attributable to obesity, and both of these cancers are becoming more common as populations have increased obesity3. These results demonstrate how sugar consumption leading to increased body fat does appear to impact on the risk of developing cancer. There are many ways that obesity contributes to cancer risk, and for people with a cancer diagnosis there are several ways that having higher body fat appears to increase cancer progression3. After a cancer diagnosis, being obese is linked to poorer survival for people with several different cancer types, including bowel, prostate and breast cancers4. So the first way that sugar may affect cancer is by increasing body fat and increasing obesity.


Sugar, Insulin and Diabetes

When we eat sugar it can be taken up rapidly by the blood in the network of blood vessels in the wall of the digestive system. Pure glucose (dextrose) in energy drinks, for example, hits the bloodstream very quickly, hence the use of glucose by athletes to fuel their intense activity. Sucrose and other sugars such as fructose and lactose also release glucose rapidly into the bloodstream. So sweetened foods such as cookies, candy and sweetened breakfast cereals provide a quick boost to the blood glucose level. Starchy foods such as bread, rice, pastry and root vegetables are digested to release sugars, and the speed of digestion determines how quickly their payload of sugar is delivered. Highly refined starchy foods, like white bread, processed breakfast cereals and fried potatoes, deliver sugar rapidly. Conversely, unrefined starches like wholegrain bread, oatmeal and brown rice, deliver their sugar in a steadier stream. This is largely because the foods contain lots of fiber and it takes the body longer to extract sugar when it is bound up with fiber.

The impact of food on blood glucose level can be measured and defined as its glycemic impact (GI) and glycemic load (GL). A general rule is that more refined starchy foods have a higher GI compared with unrefined foods, but the composition of the overall meal and differences between individuals also have an impact.

Blood sugar levels are managed by insulin, which is produced when blood sugar rises. People with high levels of insulin, and with type 2 diabetes, are at greater risk of developing many types of cancer, and a greater risk of recurrence and progression of those cancers5.

Even without a diagnosis of diabetes, having a higher level of insulin may be linked to faster progression of some cancers. A study following two thousand people affected by colorectal cancer showed that prognosis was better in people with lower insulin levels6. Another study of diet and colorectal cancer showed longer survival in people whose diet had a lower glycaemic load (lesser impact on blood sugar and insulin). Overall then, higher levels of insulin induced by a higher sugar intake appear to be linked to poorer outlook for people with many cancers7.

Learn more about how sugar and diet affect cancer

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Sugar and cell metabolism

One hotly debated aspect of sugar and cancer is the suggestion that sugar directly feeds cancer cells. As long ago as 1920, biochemists have been intrigued by the changes in cell chemistry that happen in cells that become malignant8. Otto Warburg first reported that tumor cells use glucose in large amounts and appear to have impaired metabolism, requiring much larger amounts of sugar to sustain growth, compared with normal healthy cells. In fact, when most PET scanning is carried out to help with cancer diagnosis, a specially labelled glucose is injected because the cancer cells absorb glucose much faster than healthy cells, and this shows up the tumors on the scan.

This often leads people to claim that “sugar feeds cancer”, and, in our view, to some extent it does, but that is only part of the story.  There are many other ways that cancer cell metabolism is deranged,7 for example greater use of some amino acids, which are the building blocks of protein.8  So for these reasons, some cancer researchers and practitioners suggest that dietary sugar is insignificant in feeding cancer cells. It can also be argued that if a cancer cell is deprived of sugar it will eventually find another fuel.

On the other side of the debate are research groups seeking drugs to block sugar access to cancer cells. There are also clinicians studying special diets that have much less sugar, to reduce that primary fuel for cancer cells. Trials of low carbohydrate diets have recently begun with people living with cancer, especially certain types of brain tumor9. These special diets are not for everyone, and they need expert guidance and medical monitoring, for safety as well as to enable long term use.

It is not entirely clear why sugar potentially helps cancer to grow but one mechanism suspected to be involved is that of growth factors, such as insulin-like growth factor (IGF1).  These growth factors are thought to promote cancer by accelerating cell growth – they’re a bit like a turbo on a car.  It is well accepted that higher levels of blood sugar mean higher levels of insulin (the body’s mechanism for controlling blood sugar), which in turn, so the reasoning goes, leads to higher levels of IGF1 and accelerated cancer growth.  We discuss growth factors in [Episode 5 of our Video Series].

Sugar and the gut microbiome

A truly revolutionary area of cancer research has opened up in the last decade. The microbiome is the collective of microbes that live on us and especially in us. Inside the digestive system is a collection of trillions of microbes, several pounds in a typical adult, and these have an important influence on health. It is now understood that various species within the microbiome interact with blood cells circulating in the walls of the intestine, and this interaction affects the immune system. The microbiome is extremely complex, dynamic and generally not well understood.  For example, it is not fully understood how dietary components like sugar can influence this ‘microbiome’ in cancer patients and people at risk of cancer. Studies with cancer patients receiving some new immune therapy cancer drugs seem to suggest that a healthful diet with plenty of fibre may help some cancer patients to respond particularly well to these treatments10. Much more research is needed to understand these mechanisms and how they can be optimized. It is clear that diets containing highly processed foods and few vegetables are linked to a less healthy microbiome, and certainly more cancers of the digestive system10. This is partly because of the lack of plant fibre to feed the ‘good’ bacteria, but in some laboratory studies, dietary sugar stopped some beneficial bacteria establishing themselves in the gut11. The microbiome research looking at the diet-cancer link presents a whole new angle to the question “does sugar feed cancer?”

Putting together your own healthful eating plan

Putting together your own healthful eating plan

So, finally, let’s look at how people affected by cancer can make sense of the debate, and make sensible decisions on sugar. We’ve seen how a sugar in the diet may have a range of adverse effects. As a ‘best bet’ approach, it’s wise to consider how your sugar intake and your diet overall affects your body weight. Consider also blood sugar levels, and because sugar and starchy food (‘carbs’) contribute most to blood sugar levels, try to keep your intake of added sugar low, and keep starchy foods to small portions of slow-releasing types like wholegrain foods and vegetables. Take professional advice if you want to consider cutting back further on starchy foods. Consider how sugar may be having an adverse effect on your gut microbiome. Remember that unprocessed plant foods are especially helpful to feed you and feed your beneficial bacteria.


The relationship between sugar and cancer is complex, with some research provides strong evidence and some areas are still being explored and debated. It’s clear that sugar consumption in most populations is high and linked to health problems, including cancer.

What’s important to understand is that, while more evidence is needed to definitively say that sugar promotes cancer growth, there is enough evidence now to say that patients looking to improve their position, and those looking to reduce their cancer risk, would be prudent to reduce their consumption of sugar.  This is a key point and lies at the heart of how we deal with uncertainty when addressing issues at – patients need to act on imperfect information and should not require scientific certainty before acting, particularly where there are few, if any, side effects associated with the action being contemplated.   

Johnson, Richard J., et al. "Perspective: a historical and scientific perspective of sugar and its relation with obesity and diabetes." Advances in Nutrition 8.3 (2017): 412-422
Weihrauch-Blüher, Susann, Peter Schwarz, and Jan-Henning Klusmann. "Childhood obesity: increased risk for cardiometabolic disease and cancer in adulthood." Metabolism 92 (2019): 147-152
Bhaskaran, Krishnan, et al. "Body-mass index and risk of 22 specific cancers: a population-based cohort study of 5· 24 million UK adults." The Lancet 384.9945 (2014): 755-765
Font-Burgada, Joan, Beicheng Sun, and Michael Karin. "Obesity and cancer: the oil that feeds the flame." Cell metabolism 23.1 (2016): 48-62
Cignarelli, Angelo, et al. "Diabetes and cancer: pathophysiological fundamentals of a ‘dangerous affair’." Diabetes research and clinical practice 143 (2018): 378-388
Yuan, Chen, et al. "Influence of dietary insulin scores on survival in colorectal cancer patients." British journal of cancer 117.7 (2017): 1079
Sieri, S., et al. "Dietary glycemic index, glycemic load, and cancer risk: results from the EPIC-Italy study." Scientific reports 7.1 (2017): 9757
Jeon, Sang-Min, and Nissim Hay. "Expanding the concepts of cancer metabolism." (2018): 32
van der Louw, Elles JTM, et al. "Ketogenic diet treatment as adjuvant to standard treatment of glioblastoma multiforme: a feasibility and safety study." Therapeutic Advances in Medical Oncology 11 (2019): 1758835919853958
Pryor, Rosina, et al. "The Role of the Microbiome in Drug Response." Annual review of pharmacology and toxicology 60 (2019)
Klement, Rainer J., and Valerio Pazienza. "Impact of Different Types of Diet on Gut Microbiota Profiles and Cancer Prevention and Treatment." Medicina 55.4 (2019): 84
Townsend, Guy E., et al. "Dietary sugar silences a colonization factor in a mammalian gut symbiont." Proceedings of the National Academy of Sciences 116.1 (2019): 233-238

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